1. Field of the Invention
The present invention relates to image data enhancement apparatus and methods, and more particularly to apparatus and methods of filtering undesired shading effects from video images.
2. Description of the Prior Art
In the prior art there are many image data processing systems available. Some examples of systems representative of the prior art are the following.
U.S. Pat. No. 3,621,129 to Fisher relates to devices for enhancing images by quantizing picture element levels into binary digital signals employing a threshold which is derived from the gray scale values of the image under analysis. The apparatus of the patent examines gray scale values of picture elements immediately preceding and succeding a feature under analysis and a variable threshold is generated which is derived from a mean value between the maximum white signal level and the maximum black signal level immediately before and after the feature under analysis.
The patented system does not acquire a histogram of distribution of gray scale intensity levels in the entire image to provide a uniform correction but rather provides only a localized threshold calculation.
U.S. Pat. No. 3,979,555 to Oppitek relates to a histogram equalized system that adaptively redistributes the intensity levels of video signals in order to provide an equal number of elements at all display intensity levels.
This patented system does not relate generally to a method of correcting shading effects in video images where a correction value is calculated from the variations in background level in the entire image.
U.S. Pat. No. 3,800,078 to Cochran teaches a digitally compensated scanning system wherein an initial scan of black or background information is stored in a look-up table and compared to desired data signals to eliminate undesired variations or noise resulting from photodiode leakage current or other noise sources.
This patent does not show a method of correcting shading effects in video images in which a histogram of distribution of gray scale intensity levels is acquired and used to calculate a threshold value and a range of permissable background gray scale values that generates a correction signal which when combined with the raw data provides an output signal that is corrected for undesired shading effects.
U.S. Pat. No. 4,232,400 to Yamamoto shows an echo control system for an adaptive echo canceller which cancels an echo signal while successively estimating the transmission characteristic of an echo path that includes means for filtering by convolution a transmitted signal including echo characteristics.
The patent does not teach a method of correcting shading effects in video images using a histogram of distribution of gray scale intensity levels for calculating maximum white, maximum black and threshold values and generating a correction signal to eliminate shading effects.
An article entitled "Digital Processing Techniques for Encoding of Graphics" by Ting and Prasada, which appeared in the Proceedings of the IEEE, Vol. 68, No. 7, July 1980 at page 757 and following, provides a survey of techniques for preprocessing input documents having noise contents which create poor data compression ratios.
The article discusses a piece-wise local approach to shading correction in which a number of small overlapping samples or windows are examined and a threshold is established for each of these windows. The approach presents a potential problem with respect to different thresholds at either side of a boundary between adjacent windows. Discontinuities in the threshold going from window to window may result in undesired effects in the image. The article does not present a global shading correction algorithm in which the shading for the entire image is corrected without discontinuity.
Other articles discussing the general problem of thresholding are found in IEEE Transactions on Systems, Man, and Cybernetics, Vol. SMC-8, No. 8, August 1978, at pps. 622-632. The segmentation of an image into background and foreground regions is discussed as well as the general technique of background sampling in the region of an object and the difficulties in the choice of background samples.
However, a simple and effective method and means for correcting shading effects in video images is not found among this art.
It will therefore be seen that the prior art discussed above neither teaches nor suggests the present invention as disclosed and claimed herein.